The use of bioprocessing to treat waste or waste contaminated material is well-documented. Most refineries in the U.S. have for years been operating land treatment facilities to treat American Petroleum Institute (API) separator sludges, Dissolved Air Flotation (DAF) separator floats, and other petroleum contaminated material. In the U.S., bioprocessing is being utilized to remediate and/or treat several hazardous waste sites contaminated with hydrocarbon material. In the February, 1990 symposium proceedings (EPA--Industry Meeting on Environmental Applications of Biotechnology), the EPA noted that biotechnology has been successfully utilized to treat soils and sludges from 30 to 40 wood preserving sites and that over 200 organic contaminants have been treated successfully using a combination of treatment approaches. Bioprocessing has been used to successfully remediate superfund sites which include contaminants from multiple and varied sources.
Economic and environmental considerations indicate that bioprocessing technologies offer a significant potential for the remediation and treatment of waste and waste contaminated materials. It may be noted in this connection that the areas contaminated with hazardous wastes are usually very large, requiring the treatment of large amounts of solids to meet remediation objectives. The use of ultimate disposal technologies such as incineration or chemical fixation and encapsulation results in very large expenditures of capital, in addition to the liability associated with the handling and transport of these materials to disposal areas. Biodegradation methods entail a lower cost relative to most other approaches because they are conducted on-site and use less complicated equipment. Furthermore, they can be conducted using a combination of above-ground and in situ treatments for a total treatment approach. Biodegradation methods return to the contaminated areas natural microorganisms which, once established, will continue the remediation of the site for years to come. The natural cleansing properties of most waste sites are stimulated by biodegradation methods, which in time benefit those portions of a site which would not normally be treated by alternative approaches (e.g., soils within the vadose zone).
Bioprocessing involves exploiting abilities of indigenous or augmented microorganisms to metabolize organic substrates. This process is beneficial as a remediation option if, through metabolism, the toxic constituents in the contaminated media are converted to non-toxic constituents or their concentration is reduced such that they no longer pose a threat to the human health and environment. Bioprocessing can be accomplished in a land-based environment (e.g., landfarming, composting); it can be performed in tanks (e.g., tank-based groundwater treatment, slurry-phase bioremediation); it can be accomplished in situ by enhancing microbial degradation of contaminants in the subsurface soil; it can be completed under aerobic and/or anaerobic environments; and it can utilize either indigenous or cultured/augmented microorganisms.
Depending upon site specific conditions, processes other than microbial degradation, e.g. volatilization, adsorption, and photodegradation also will take place during bioprocessing. Such physical phenomena tend to put in question the effectiveness of bioprocessing when applied without the use of emission controls and chemical stabilization of treated residues. For the biodegradation process to succeed, at least three criteria must be met. First, a microbial community possessing the appropriate metabolic capacity to effect complex biodegradations must be present. The presence of such a microbial community rests upon prior exposure of these and other naturally occurring microorganisms to similar materials. Second, microorganism-substrate interaction is required, which depends upon the bioavailability of the potential substrate. Finally, environmental parameters such as temperature, pH, availability of oxygen, nutrients and moisture must be conducive to the growth of microorganisms.
A relatively recent bioprocessing approach used to treat contaminated soils and sludges containing certain wood preservatives (e.g., pentachlorophenol), some biodegradable herbicides, and selected hydrocarbon material (e.g., petroleum-based oils and greases) is slurry-phase bioremediation (U.S. EPA Engineering Bulletin--Slurry Biodegradation, EPA/540/2-90/016). This technology is usually a tank-based bioprocessing method that is sometimes referred to as a liquids/solids system. Slurry-phase bioremediation processes treat organic sludges and contaminated soils by extraction and biodegradation. It generally requires extensive, high-power mixing to suspend solids in the slurry-phase and to maximize the mass transfer of the organic contaminants to the aqueous phase where biodegradation normally occurs.
Slurry-phase bioremediation processes generally provide more rapid treatment and require less area than such bioprocesses as landfarms, soil heaps, and compost piles. The residence time in the slurry-phase bioremediation process varies with the waste constituent matrix, physical/chemical properties, contaminant concentration, and constituent biodegradability. Conventional slurry-phase bioremediation processes have the following characteristics:
(a) biodegradation always occurs under aerobic conditions;
(b) batch processing has been the most common mode of operation;
(c) aeration and mixing is generally provided by floating mechanical aerators or high-speed (e.g. 300-800 rpm) mechanical turbines with submerged aeration;
(d) synthetic chemical-based surfactants and dispersants are usually added to achieve waste constituent dissolution; and
(e) volatilization rather than biodegradation is typically a major contributor to literature reported waste constituent removals for many contaminant categories (e.g., petroleum aromatics, purgeable halocarbons, polynuclear aromatic hydrocarbons).
Conventional slurry-phase bioremediation processes have decidedly limited waste treatment potential because of the following:
(a) Operation under batch conditions places a small number of unacclimated microorganisms in contact with a waste at its highest pollutional strength. Conventional processes can only compensate by diluting the waste with water and increasing the mixing energy, which increases the amount of aromatic constituent volatilization during treatment.
(b) Addition of synthetic chemical-based surfactants and dispersants often results in microbial inhibition which reduces the biodegradation potential of the process.
(c) The use of floating mechanical aerators and/or high-speed mechanical turbine mixers is very energy intensive, which usually makes operation at high slurry densities (i.e., greater than 25% solids by weight) uneconomical.
(d) The potential for a high degree of waste constituent volatilization suggests the need for air pollution controls on the process. Typical air emission control approaches include vapor phase activated carbon adsorption and/or fumes incineration which adds substantial cost to the process.